Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (for example balls and/or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling contact fatigue, wear and creep.
Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming/machining process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished or semi-finished components so as to improve, for example, wear resistance and fatigue resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue resistance.
An alternative to case-hardening is through-hardening. Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example.
For through-hardened bearing steel components, two heat-treating methods are available: martensite hardening or austempering. Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat-treatment employed.
The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be low-temperature tempered to stabilize the microstructure.
The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels instead of martensite through-hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance.
Bainitic steel structures are produced by the transformation of austenite to bainitic-ferrite at intermediate temperatures of typically from 190 to 500° C. The cooling of the austenite leads to a microstructure comprising ferrite, carbides and retained austenite. Bainite itself comprises a structure of supersaturated ferrite containing particles of carbide, the dispersion of the latter depending on the formation temperature. The hardness of bainite is usually somewhere intermediate between that of pearlite and martensite.
The steel known as SP10 has the following chemical composition: Fe-0.8C-1.5Si-2Mn-1Al-1Cr-0.25Mo-1.5Co in wt. %. Austenitisation followed by bainite hardening (200° C., 72 hours) results in a fine microstructure comprising retained austenite and bainitic ferrite. However, the hardness and dimensional stability of this alloy structure are deemed too low for bearing applications.
It is an object of the present invention to address some of the problems associated with the prior art, or at least to provide a commercially useful alternative thereto.